This commit gives the `lfs.Hook` knowledge of the new `core.hooksPath`
configuration value that was introduce in Git 2.9.0.
When `core.HooksPath` is found in the Git configuration AND is supported (i.e.,
installed Git binary has a version greater than or equal to "2.9.0"), `git-lfs
install` will place new hooks in that directory. If the `core.hooksPath` is
specified, but the installed verison of Git does NOT support it, then it will
be ignored and installed in `.git/hooks` as per usual.
To test this behavior, two new shell tests were added:
- One that runs on Git >= 2.9.0, and tests that `core.hooksPath` is respected
- One that runs on Git < 2.9.0, and tests that `core.hooksPath` is ignored
Unfortunately, our current testing framework does not support skipping
individual tests, only skipping entire files, so two new shell test files were
added.
Resolvesgithub/git-lfs#1407.
